Electricity and magnetism · GCSE Physics
Electricity
The flagship circuits page: current, potential difference, resistance, series and parallel, I–V characteristics, power and the national grid.
Current is the flow of charge. Potential difference is the energy per charge. Resistance is how hard it is to make that charge flow. Series shares the current; parallel shares the potential difference.
The important bits
What you need to know
- 1
Current I = Q / t (amperes, coulombs, seconds). Conventional current is from positive to negative. In metals the moving charges are electrons, which actually drift the other way.
- 2
Potential difference V = E / Q. Resistance R = V / I. Ohm’s law holds for an ohmic conductor at constant temperature: I is proportional to V.
- 3
In series: current is the same everywhere; potential differences add; total resistance is the sum of the resistances. In parallel: potential difference is the same across each branch; currents add; total resistance is less than the smallest branch.
- 4
Filament lamps and diodes are non-ohmic. A lamp’s resistance rises as it heats. A diode lets current flow mainly in one direction. Thermistors decrease in resistance as temperature rises. LDRs decrease in resistance as light intensity rises.
- 5
Power P = VI = I²R = V²/R. Energy transferred electrically E = Pt = VIt. Mains in the UK is about 230 V a.c. at 50 Hz. Cells and batteries supply d.c.
- 6
The national grid uses step-up transformers to increase potential difference and decrease current for transmission, which reduces heating (I²R) in the cables, then step-down transformers for safe domestic use. Transformers need a.c.
- 7
A three-core cable has live, neutral and earth. The fuse or circuit breaker sits in the live wire and melts or trips if current is too high. The earth wire and a plastic or earthed metal case protect the user if a fault makes the case live.
- 8
Static electricity is a build-up of charge, usually by friction transferring electrons. Like charges repel; unlike charges attract. Sparking is a risk near flammable fuels; earthing removes excess charge.
Go deeper
Series versus parallel is the whole circuit topic
If there is only one loop, it is series: the current has nowhere to split, so an ammeter reads the same in every gap. The cell’s potential difference is shared, and a bigger resistor takes a bigger share. Add a resistor in series and the total resistance rises, so the current from the cell falls. If there are junctions, it is parallel: each branch sees the full cell potential difference, and the currents in the branches add to the current in the main loop. Add another identical branch and the total resistance falls, so the current from the cell rises. That is why fairy lights in series all go out when one bulb fails, and why house lights are in parallel — each lamp can be switched alone, and each gets 230 V.
Go deeper
I–V graphs are resistance stories
A straight line through the origin means R is constant: a resistor at steady temperature. A curve that gets shallower as V increases is a filament lamp: temperature up, resistance up, so extra V buys less extra I. A diode is almost no current one way, then a sharp rise the other way after a threshold. Thermistor in a potential divider: cold means high resistance, so a large share of the p.d.; hot means low resistance. LDR: dark, high R; light, low R. In the required practical, use a voltmeter in parallel with the component, an ammeter in series, a variable resistor or variable supply to change V, and do not leave a lamp on so long that the temperature is uncontrolled if you are trying to test Ohm’s law.
Go deeper
Power, fuses and the grid are the same I²R idea
Heating in a wire is I²R. A fuse is a thin wire designed to melt at a stated current, breaking the live connection. Choose the fuse a little above the normal operating current: a 3 A fuse for a 2.5 A lamp, not 13 A, or the cable could overheat first. The grid raises V so that for the same power, I falls, so I²R losses in the cables fall. That is why transmission is at hundreds of kilovolts, not at 230 V. Transformers only work with a.c., which is why the mains is alternating. Energy transferred to a home is still P × t; the bill uses kilowatt-hours because a joule is too small to price. None of this changes the circuit rules: live to the fuse and switch, then to the appliance.
See the idea in action
A 12 V battery supplies a 6 Ω resistor in series with a 3 Ω resistor. Total R = 9 Ω. Current in the loop I = V/R = 12/9 = 1.33 A (same through both). p.d. across 6 Ω = IR = 1.33 × 6 ≈ 8.0 V. Across 3 Ω ≈ 4.0 V. They add to 12 V. Power in the 6 Ω resistor = I²R ≈ 1.78 × 6 ≈ 10.7 W. If the 3 Ω resistor is instead connected in parallel with the 6 Ω across 12 V, each branch sees 12 V. Currents are 12/6 = 2.0 A and 12/3 = 4.0 A. Total current from the battery = 6.0 A, much larger, because total resistance has fallen.
Exam technique
Turn knowledge into marks
Sketch the circuit and mark where current splits before you calculate. Series: I the same, V adds, R adds. Parallel: V the same, I adds. For lamps and diodes, say whether resistance is constant. For mains safety, name live, fuse, earth and the fault they protect against. Show P = VI or I²R with units.
Common mistakes
Do not give these marks away
- 01
Putting an ammeter in parallel or a voltmeter in series, or saying current is used up in a resistor.
- 02
Adding resistances in parallel as if they were in series, or saying the mains is d.c.
- 03
Choosing a 13 A fuse for every appliance, or explaining the national grid without lower current and less heating.
In a series circuit, which quantity is the same through every component?
APotential difference
BResistance
CCurrent
DPower
Show the answer
Current. There is only one path, so charge cannot pile up. Current is the same at every point. Potential difference is shared and power need not be equal.
Quick questions
If this is the bit you searched
What is the difference between series and parallel circuits?
Series: one loop, same current, p.d. shared, resistances add. Parallel: branches, same p.d. across each branch, currents add, total resistance falls as branches are added.
Why does the national grid use a high potential difference?
For a given power, higher V means lower I. Lower current reduces heating losses in the cables (I²R) so more of the generated energy reaches homes.
What does an LDR do?
A light-dependent resistor. Its resistance decreases as light intensity increases, so it can be used in potential dividers for lighting circuits.